I'll be honest, as a mage, I like to start fights with 100% mana. It doesn't matter if it's a trash pull or boss pull.
In theory, you never know when you're going to get an extra mob by accident or let the mob get off an extra heal before the interrupt (see also: kill). A mage (and a shadow priest for that matter) without mana can do a maximum of 184 DPS (non-crits) and that's with one of two Black Temple drops. You want to get less crazy, you can get the [Merciless Gladiator's Touch of Defeat], but again, that's only 179 DPS. Since mages have Evocation (every 8 minutes) and mana gems (every 2 minutes), for this analysis, I'd probably group mages with Moonkin Druids (Innervate every 6 minutes) or Shadow Priests (Shadowfiend every 5 minutes). Other classes can life tap (assuming a healer still has mana) or still sustain some dps through weapons or auto-shot.
In practice, however, I think I rarely use more than 50% mana on typical trash pulls. And the 50% is probably only when we have heavy AoE fights. So, in reality, I really don't need to have much down-time between each of the trash fights. I can drink while the trash is being pulled and controlled (assuming no sheep), which should be sufficient to sustain my mana to acceptable levels if not full. I tend to save the evocation for the boss fights because of the long cooldown; but, worst case scenario, I could use that on trash mobs too if need be.
I bring this all up because of what we are now doing in Karazhan. We've pretty much distributed the people into two hopefully equal teams. And both teams move very quick. We did through Curator last night, and then tonight should be another semi-easy walk in the park to clear the rest. It's fast paced, so it feels a lot like the 45-minute Baron runs. Which leads me to think... why were we going slow before? Yeah... I'd say we have Karazhan on farm status.